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Hydrodynamic studies of spouted and spout-fluid beds

University of British Columbia

Hydrodynamic experiments were conducted in a three-dimensional spout-fluid bed with a specially designed conical distributor to supply the auxiliary air. The vessel used was a cylindrical column, 0.15 m in diameter and 1.05 m high fitted with an inlet orifice plate with a diameter of 19.1 mm. Glass beads of three different mean diameters, 1.33, 1.84 and 2.53 mm, were used as bed materials. Previously developed techniques employing optical fibre sensors and instruments were used to measure local time-averaged voidage and particle velocity. Radial profiles of local voidage inside the bed were obtained for various ratios of auxiliary air flow to total air flow (Q^/QT) to the column. Increasing the proportion of auxiliary flow at constant total flow resulted in a significant decrease in spout voidage, while little or no influence was observed in the annulus. Voidage in the annulus varied significantly from the bottom to the top of the bed. In the conical section, local voidages were consistently lower than the loose-packed voidage and consistently higher than the loose-packed voidage in the cylindrical section. The low voidage in the conical section suggests that particles are being compacted in this region. Cross-sectional average voidages in the spout decreased monotonically with height and were lower for a higher proportion of auxiliary flow but independent of particle size. Spout diameter was unaffected by the proportion of auxiliary air being supplied to the column, supporting the findings of Sutanto (1983). Both the McNab (1972) and Wu (1986) equations were found to under-predict average spout diameters by as much as 28%. In general, particle velocity decreased with increasing proportion of auxiliary flow. However, the effect was more pronounced in the spout than in the annulus. Solids mass flow rates decreased with increasing proportion of auxiliary flow. The observed increase in solids mass flow rates with particle size is most likely due to the higher gas velocities required for spouting. The integrated upward solids flow in the spout was consistently higher than the corresponding downward solids flow in the annulus. The discrepancy can be attributed to inherent inaccuracies in the measurement techniques and instruments used as well as physical phenomena which could not be entirely eliminated. Pressure gradients in the annulus increased with an increasing proportion of auxiliary gas, as expected. The effect of auxiliary air was greater at the bottom of the bed, and the profiles converged towards a maximum value at the top of the bed. In addition, discontinuities in the profiles were observed near the cone-cylinder junction. Superficial gas velocities in the annulus were obtained by combining the pressure gradients with measured local voidage in an equation of the form of the Ergun (1952) equation for fluid flow through a packed bed. The results were not entirely consistent, most likely due to the sensitivity of the calculation to voidage measurements which had considerable scatter.

Thesis/Dissertation

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2009-03-06

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http://hdl.handle.net/2429/5696

Chemical and Biological Engineering

University of British Columbia

UBCV

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